One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin layer on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to generally as chemical-vapor deposition (“CVD”). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired layer. Plasma-enhanced CVD (“PECVD”) techniques, on the other hand, promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared to conventional thermal CVD processes. These advantages are further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive. “High-density” is understood in this context to mean having an ion density that is equal to or exceeds 1011 ions/cm3.
During a conventional chemical vapor deposition process, the substrate remains on the surface of the pedestal or support structure for the duration of the deposition and is then lifted off the pedestal by a lifting mechanism for output from the chamber. The lifting mechanism may comprise, for example, a servo-driven motor attached to a plurality of lift fingers which minimally contact the bottom surface of the substrate. In some deposition processes, particles on the backside of the substrate may be removed after the deposition process by lifting the substrate to an elevated position and exposing the substrate to a plasma to clean the substrate and remove backside contamination.
In a relatively newly developed method of enhancing transistor performance, the atomic lattice of a deposited material is stressed to improve the electrical properties of the material itself, or of underlying or overlying material that is strained by the force applied by a stressed deposited material. Lattice strain can increase the carrier mobility of semiconductors, such as silicon, thereby increasing the saturation current of the doped silicon transistors to thereby improve their performance. For example, localized lattice strain can be induced in the channel region of the transistor by the deposition of component materials of the transistor which have internal compressive or tensile stresses. For example, silicon nitride materials used as etch stop materials and spacers for the silicide materials of a gate electrode can be deposited as stressed materials which induce a strain in the channel region of a transistor. The type of stress desirable in the deposited material depends upon the nature of the material being stressed. For example, in CMOS device fabrication, negative-channel (NMOS) doped regions are covered with a tensile stressed material having positive tensile stress; whereas positive channel MOS (PMOS) doped regions are covered with a compressive stressed material having negative stress values.
As tensile and compressive stresses are examples of internal loading, they may be regarded as positive and negative values, respectively, of the same type of normal loading. Thus, an unstressed material is neither compressive or tensile. A material may progress from having a compressive stress to becoming more tensile and gradually exhibiting a tensile stress depending on external factors, and vice versa.
Given the stresses created by depositing material on a substrate, it is desirable to control the level of stress generated in the deposited material, as well as change the level of stress after processing steps are complete. A variety of different deposition parameters can control the stress level of a material during deposition, including temperature, and RF power levels among others. Additionally, various techniques have been developed to change the stress of a material already deposited over a substrate including exposure of the substrate to a plasma, exposure of the substrate to ultraviolet light or electron beams, and annealing the deposited layer. Despite the availability of the above techniques, new methods of changing the level of stress of a deposited layer are desirable.